1. Technical Field
The present application relates to a nitride-based semiconductor light-emitting element which includes a substrate which has a principal surface, a rear surface that is a light extraction surface, and a plurality of lateral surfaces, and a nitride semiconductor multilayer structure formed on the principal surface of the substrate. The present application also relates to a light source which includes a nitride-based semiconductor light-emitting element and to a method for manufacturing a nitride-based semiconductor light-emitting element.
2. Description of the Related Art
A nitride semiconductor containing nitrogen (N) as a Group V element is a prime candidate for a material to make a short-wave light-emitting device, because its bandgap is sufficiently wide. Among other things; gallium nitride-based compound semiconductors have been researched and developed particularly extensively. As a result, blue light-emitting diodes (LEDs), green LEDs, and blue semiconductor laser diodes in which gallium nitride-based compound semiconductors are used have already been used in actual products.
Hereinafter, the nitride semiconductors include a compound semiconductor in which some or all of gallium (Ga) atoms are replaced with at least one of aluminum (Al) and indium (In) atoms. Therefore, the nitride semiconductors are represented by formula AlxGayInzN (0≦x, y, z≦1, x+y+z=1).
By replacing Ga atoms with Al or In atoms, the bandgap can be greater than that of GaN. By replacing Ga atoms with In atoms, the bandgap can be smaller than that of GaN. This enables not only emission of short-wave light, such as blue light or green light, but also emission of orange light or red light. Because of such a feature, a nitride-based semiconductor light-emitting element has been expected to be applied to image display devices and lighting devices.
The nitride semiconductor has a wurtzite crystal structure. FIG. 1A to FIG. 1C show planes of a wurtzite crystal structure with four characters (hexagonal indices). In a four-character expression, crystal planes and orientations are expressed using primitive vectors of a1, a2, a3, and c. The primitive vector c runs in the [0001] direction, which is called a “c-axis”. A plane that intersects with the c-axis at right angles is called either a “c-plane” or a “(0001) plane”. FIG. 1A shows c-plane as well as a-plane and m-plane. FIG. 1B shows r-plane. FIG. 1C shows (11-22) plane.
FIG. 2A shows a molecular orbital model of the crystal structure of the nitride semiconductor. FIG. 2B is a diagram showing an atomic arrangement near an m-plane surface, which is observed from the a-axis direction. The m-plane is perpendicular to the drawing sheet of FIG. 2B. FIG. 2C is a diagram showing an atomic arrangement at a +c-plane surface, which is observed from the m-axis direction. The c-plane is perpendicular to the drawing sheet of FIG. 2C. As seen from FIG. 2B, N atoms and Ga atoms reside at a plane which is parallel to the m-plane. On the other hand, as seen from FIG. 2C, the c-plane includes layers in which only Ga atoms reside and layers in which only N atoms reside.
According to the conventional techniques, in fabricating a semiconductor element using nitride semiconductors, a c-plane substrate, i.e., a substrate which has a (0001)-plane principal surface, is used as a substrate on which nitride semiconductor crystals are to be grown. In this case, due to the arrangement of Ga atoms and N atoms, spontaneous electrical polarization is produced in the c-axis direction in the nitride semiconductor. That is why the c-plane is also called a “polar plane”. As a result of the electrical polarization, a piezoelectric field is generated along the c-axis direction in the InGaN quantum well in the active layer of the nitride-based semiconductor light-emitting element. This electric field causes some positional deviation in the distributions of electrons and holes in the active layer, so that the internal quantum yield decreases due to the quantum confinement Stark effect of carriers.
Thus, it has been proposed that a substrate of which the principal surface is a so-called “non-polar plane”, such as m-plane or a-plane, or a so-called “semi-polar plane”, such as −r plane or (11-22) plane, be used. As shown in FIG. 1, the m-planes in the wurtzite crystal structure are parallel to the c-axis and are six equivalent planes which intersect with the c-plane at right angles. For example, in FIG. 1A, the (1-100) plane that is perpendicular to the [1-100] direction is the m-plane. The other m-planes which are equivalent to the (1-100) plane include (−1010) plane, (10-10) plane, (−1100) plane, (01-10) plane, and (0-110) plane. Here, “−” attached on the left-hand side of a Miller-Bravais index in the parentheses means a “bar”, which conveniently represents inversion of that index.
On the m-plane, as shown in FIG. 2(b), Ga atoms and N atoms are on the same atomic-plane. For that reason, no electrical polarization will be produced perpendicularly to the m-plane. Therefore, if a light-emitting element is fabricated using a semiconductor multilayer structure which has been formed on the m-plane, no piezoelectric field will be generated in the active layer, thus overcoming the problem that the internal quantum yield is decreased due to the quantum confinement Stark effect of carriers. This also applies to the a-plane that is one of the other non-polar planes than the m-plane. Also, similar effects can be achieved even in the case of a so-called semi-polar plane, such as −r plane or (11-22) plane.
Further, a nitride-based semiconductor light-emitting element including an active layer which is formed on the m-plane, the a-plane, the −r plane or the (11-22) plane has a polarization characteristic which is attributed to the structure of its valence band.
For example, Japanese Laid-Open Patent Publication No. 2009-71174 (hereinafter, referred to as “Patent Document 1”) discloses, as a method for improving the polarization characteristic of a nitride-based semiconductor light-emitting element whose principal surface is a non-polar plane or a semi-polar plane, separating nitride-based semiconductor light-emitting elements into small chips each having a rhombic shape or a triangular shape, so that a resultant configuration prevents decrease of the polarization degree of light outgoing from a lateral surface of the nitride-based semiconductor light-emitting element.
For example, Japanese Laid-Open Patent Publication No. 2007-234908 (hereinafter, referred to as “Patent Document 2”) discloses, as a method for improving the reliability of a nitride-based semiconductor light-emitting element, isolating M-plane or R-plane nitride-based semiconductor light-emitting elements into nitride semiconductor chips in such a manner that the isolation is carried out with an inclination of 30 to 60 degrees with respect to a cleavage surface.
For example, Japanese Laid-Open Patent Publication No. 2008-277323 (hereinafter, referred to as “Patent Document 3”) discloses, as a method for improving light extraction from a nitride-based semiconductor light-emitting element having an a-plane principal surface, isolating nitride semiconductor chips in such a manner that the isolation is carried out with an inclination of 5 to 85 degrees with respect to a cleavage surface. This leads to generation of recesses and elevations across a lateral surface of the nitride semiconductor chip, so that light extraction from the lateral surface improves.